1. Field of the Invention
The present invention generally relates to equalizing circuits and, more particularly, to an equalizing circuit for amplifying bass range.
2. Description of the Related Art
In recent years, liquid crystal television sets and plasma display television sets have appeared in the market and are replacing CRT television sets. With the appearance of these television sets, increase in the screen size of a television set and reduction in the thickness thereof are facilitated. Under these circumstances, the size and installed position of an enclosure for a speaker in a television set are subject to increasingly rigid constrains. Since the size of enclosure is generally becoming smaller, reproduction of rich bass sound and creation of a sense of realism have become difficult. Generally, the human auditory system is less sensitive to bass sound. For this reason, there is likelihood that bass range sound cannot be heard unless the sound pressure is increased. There is proposed a preamplifier in which two operational amplifiers are in a cascade connection in order to amplify a bass range signal. This preamplifier is capable of amplifying a signal at 100 Hz or lower (See, for example, Reference (1) in the following Related Art List).
3. Related Art List
(1) “Microcomputer controlled audio preamplifier”, Peter Kruger, Electronics and Wireless World, June 1989, pp. 584-587
However, the related-art preamplifier amplifies signals at extremely low frequencies so that bass distortion associated with the driving of a speaker is also amplified. A band-pass filter preset to have a central frequency of 100 Hz or the neighborhood may be used to suppress bass distortion. If the band-pass filter has a high filter Q-factor, the frequency bandwidth is narrowed so that sufficient sound quality cannot be obtained. Decreasing the Q-factor of the filter affects a vocal frequency band at 300 Hz and higher. This results in both the bass range and the vocal frequency band being amplified so that sufficient sound quality cannot be obtained. An alternative approach is to separate a signal in the bass range and that of the vocal frequency band by a filter so that the signal in the bass range is amplified and then combined with the signal in the vocal frequency band afterwards. In this case, phase rotation occurs in the filter so that a dip occurs, in which the phase of the amplified signal and that of the vocal frequency band are reversed with respect to each other so that the signal components cancel each other. As a result, the sound quality is degraded.